This invention relates generally to traveling-wave tubes and more particularly to arrangements for preventing backward-wave oscillations in helix or helix-derived slow-wave interaction structures.
Helix traveling-wave tubes are widely used in commercial and military power applications where wide bandwidth is a primary requirement. The conventional helix traveling-wave tube provides the greatest bandwidth of any microwave power source but has inherent stability limitations which have prevented high power output. The tendency for backward-wave oscillations to occur is the primary deterrent to increasing the peak power attainable from helix traveling-wave tubes. The backward-wave oscillations at unwanted frequencies can cause beam modulation that substantially reduces the output power at the desired frequency. Prior art methods to reduce backward-wave oscillations in general have tended to greatly reduce the bandwidth, power, and efficiency of the traveling-wave tubes.
Prior methods of reducing the tendency for backward-wave oscillations to occur include distributed RF loss, phase velocity tapering axially along the beam, periodic circuit perturbations to produce a frequency stopband, a small beam diameter to minimize backward-wave interaction, and a short helix length between the sever and output. All of these methods have drawbacks which severely limit the tube efficiency, bandwidth, and pulse-up capability (dual-mode tube), or require excessive magnetic forcusing fields or mechanical complexity. Some types of traveling-wave tubes using helix-derived circuits such as the ring-bar circuit or the strapped bifilar helix can generate higher peak power but are severely limited in bandwidth capability and are difficult to fabricate. These helix-derived devices also often have severe stability problems associated with backward-wave oscillations or band-edge oscillations.